Organic materials have been widely used in electronic devices such as organic field-effect transistors (OFETs), light emitting diodes (OLEDs), photovoltaic diodes, and liquid crystal displays. OFETs are of special interest for use in low-cost integrated circuit (IC) technology suitable for applications such as smart cards, electronic tags, displays, and memory devices. In OFETs, the semiconductor layer consists of organic semiconductor materials including conjugated polymers and oligomers. Many organic materials possessing the required electronic properties for the electronic device applications have been synthesized.
Organic compounds that have been investigated for use as semiconductors include conjugated polymers such as regioregular poly(3-alkylthiophene)s; copolymers of polyfluorene-bithiophene; polyaromatic amine and polythiophene derivatives; fused aromatic compounds such as pentacene, tetracene and their derivatives; and conjugated oligomers such as oligothiophenes, fluorene-thiophene oligomers, and phenyl-thiophene oligomers.
The performance of most of the above organic semiconductors compounds suffers from either low charge mobility (ca. ˜0.1 cm2/Vs) or instability. For example, although pentacene possesses high charge mobility (about 0.1 to 2 cm2/Vs), it also has a relatively low band gap (2.2 eV) and a high HOMO (highest occupied molecular orbital) energy level, and is easily oxidized. Pentacene compounds often show high oxygen and humidity sensitivity, and therefore high on/off ratios can only be obtained in an inert atmosphere. These characteristics result in poor device stability and make pentacene compounds unsuitable for practical electronic circuit applications. On the other hand, compounds such as oligofluorenes, oligofluorene-thiophenes, phenylene-thiophene, and conjugated polyfluorene-thiophene polymers show improved stability, but their low charge mobility limits their applications in high efficiency electronic devices.
Therefore, there still exists a need for organic compounds that have high charge mobility and high on/off ratio, and that are stable to heat, light, and air. There is also a need for organic compounds that can be readily incorporated into electronic devices using commercially viable fabrication methods.